I'm a professional copywriter /designer/brand manager - few suggestions (sorry my German is terrible - but I understand your text in general without using the Google Translator):

1 - Always look at your site from the client's stand point: "What is it here for me?" So, put those stuff in focus.

2 - At the top of your page - put your most profitable and/or most attractive services for the clients;

3 - Do you have a service which is somehow unique or rare? Post it at the visible place (near the top of the page)

4 - colors - I like the color scheme -. but you should know that warm colors are the ones which will provoke the action. So, make those contact button in orange (for example) and your main selling messages in warmer colors. This stuff is proven to bring results....

5 - I like the "Uber uns" page - that's the way for easing your future patients to contact you - thumbs up!

That would be my first "ad hoc" impressions - good luck in your promotion!